Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium called a disc. Modern disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Each surface of a disc is divided into several thousand tracks that are tightly-packed concentric circles similar in layout to the annual growth rings of a tree. The tracks are typically numbered starting from zero at the track located outermost the disc and increasing for tracks located closer to the center of the disc. Each track is further broken down into data sectors and servo bursts. A data sector is normally the smallest individually addressable unit of information stored in a disc drive and typically holds 512 bytes of information plus additional bytes for internal drive control and error detection and correction. This organization of data allows for easy access to any part of the discs. A servo burst is a particular magnetic signature on a track, which facilitates positioning of heads over tracks.
Generally, each of the multiple discs in a disc drive has associated with it two heads (one adjacent the top surface of the disc and another adjacent the bottom) for writing and reading data to or from a sector. Each head is mounted at the distal end of an actuator arm that extends toward the disc and pivots about the bearing shaft assembly connected to a voice coil motor in the disc drive. A read element (or a reader) and a write element (or a writer) are mounted on each head. A gap separates the reader and writer along the longitudinal axis of the actuator arm. The head skew angle, which is the angle between a tangential line to a track and the line drawn along the longitudinal axis of the actuator arm, changes as the head moves from the inner diameter to the outer diameter of the disc, and vice versa. The combination of the gap and the varying head skew angle causes the radial distance between the path of the reader on the disc and the path of the writer on the disc to be variable as the head moves from the inner diameter to the outer diameter of the disc, and vice versa. This varying radial distance between the reader and the writer is known as the reader-to-writer offset.
In general, the data storage format of a track is comprised of an alternating sequence of address headers (including servo fields) and data fields on a track. The address headers store address information, which identifies the respective addresses of the data fields. The data fields store user data. Two methods are typically used to write the address headers and data fields. The first method is to write the alternating address headers and data fields as close to the center of the track as possible. The second method is to write them in alternating sequence but to write the data fields at an offset from the servo fields in order to take into account the presence of the reader-to-writer offset. The basic difference between the first method and the second method is that the first method requires a micro minijog of the actuator arm during a write operation whereas the second method requires a micro minijog of the actuator arm during a read operation. For example according to the first method, during a write operation, the reader first reads the address headers and compares them to the target address. If the address read from an address header matches the target address, the writer writes the data in the data field. However, as soon as a target data field has been identified, the actuator arm must perform a minijog to position the writer over the data field so that the writer can write data in the target data field. The reader-to-writer offset is the distance the actuator arm must displace in order to position the writer over the target data field. This micro minijog of the actuator arm to position the writer over the data fields is not required according to the second method since the data fields are already prewritten at an offset, which is substantially equivalent to the reader-to-writer offset. However, just the opposite during a read operation, the actuator arm is required to perform a micro minijog to place the reader over the data field.
This reader-to-writer offset measurement is crucial since it will impact the disc drive performance against the track misregistration (TMR). TMR generally refers to position errors of the head between the target head position and the actual head position influenced by external disturbances such as disc flutter, runouts, disc vibrations, etc. The reader can read good data (i.e., data that contains no bit error or recoverable bit errors) only on small a portion of the track pitch (or width) of the track, and this portion of the track pitch is generally referred to as the off-track capability (OTC) of the head. For example, the OTC of a disc drive may only be about 10% of the track pitch. Thus, the reader or the writer must be positioned within the OTC (i.e., within the 10% of the track pitch) in order to successfully read information from or write data to the track.
If the reader-to-writer offset measurement contains an error, one outcome is that the target head position may not be within the OTC of the head. The other outcome is that the target head position may not be located at the center of the OTC although it may be within the OTC of the head. In such a case, the target head position would still allow the reader to successfully read good data written on the track but would not provide optimal protection against the TMR. This is because the target head position would be located closer to one of the two edges of the OTC, and there exists higher probability that the external disturbance might displace the head outside the OTC of the head.
The main technique that is currently used to determine the reader-to-writer offset is known as the bit-error-rate (BER) technique; however, this technique does not require that the target head position be located at the center of the OTC of the head. Accordingly, there is a need for determining an optimal reader-to-writer offset value that allows the target head position to be located at the center of the OTC of the head and provides better protection against TMR.